Process for the separation of cadmium from its ores



Patented Dec. 22, 1936 UNITED sT-ArE-s PROCESS FOR THE SEPARATION OF GADMIUM FROM rrs ORES Reed w. Hyde, Summit, N. J.

No Drawing.

Application August 8, 1935, Serial No. 35,343

1 Claim. (01. 75-1) My invention relates to a process for separat- 4 ing and recovering cadmium from ores containing relatively large amounts of other metals.

Cadmium is usually found in nature associated 5 with other metals, principally zinc, but often with small amounts of lead, copper and other metals. The principal sources of cadmium are ores in which zinc is the predominating metal as to value and in which cadmium occurs in minor quanti io ties. Heretofore the ores have generally been first roasted to remove the sulphur as completely as possible, and then treated for reduction of the metals to bullion, after which the bullion is refined by various methods to separate the several metals. These refining operations are diflicult and costly, especially when a number of different metals are present. In many cases the metals present in smaller proportions are largely lost in endeavoring to recover the principal metal, or

the cost of recovering them may be so great asto exceed their value.

Among other methods of separating the metals has been the use of selective or fractional volatilization to obtain a fume of different metal content from that of the original material treated.

Generally, however, the fumes obtained heretofore have been troublesome to handle and exces-. sive for the quantity of cadmium contained therein.

My present invention provides a process in which cadmium may be efficiently separated from other metals in a fume that is small in volume and easily handled.

In my present invention I treat the ore or ore l5 concentrate with a fluoride in quantity just sufllcient or slightly in excess of that required to combine with the cadmium to form a cadmium fluoride. If other metals are desired in the fume, the quantity of fluoride added may be increased l0 proportionately to the quantity or quantities of such metals to be. separated with the cadmium. In treating cadmium containing zinc sulphide ore or similar ore, it is generally preferable to roast the ore before treatment by my process so as to remove all or a large part of the sulphide sulphur and to convert the ore principally to an oxide.

Also it is generally preferable to remove all gangue" which is volatilized and is removed and collected as fume.

Any suitable fluoride may beused as, for example, calcium fluoride, fluorspar, or an alkali fluoride such as sodium fluoride. Any suitable 5 way of roasting the mixture of fluoride and ore may be employed, but it is preferably roasted by moistening, forming into small masses and roasting in a sintering furnace.

By way of example, the invention will be de- 10 scribed in connection with the treatment of an ore containing cadmium with large proportions of zinc and minor proportions of lead and copper, although it will be'understood that it may also be applied to other ores.

The ore may first be concentrated to remove as much of the gangue as possible or, if it be treated as crude ore, it may be crushed to a suitable size. The, most suitable size depends to some ex-, tent on the character of the ore, but usually 0 crushing to pass a four mesh or eight mesh screen will be sufficient. The crushed ore may then be roasted to remove the sulphur, the degree of roasting depending on the value of the sulphur. For example, if sulphuric acid is made from the '25 roaster gases,itmay be desirable, from an eco-' nomic standpoint, to roast the ore down to two or three per cent sulphur, which may require the burning of external fuel in the latter stage of the roasting operation. But if the sulphur gases 30 h are not to be utilized, or if the value of the acid does not warrant the extra cost of carrying the roasting so far, this operation may be stopped with six to eight per cent sulphur remaining in the product. This roasting operation may be, conductedin any standard type of roasting furnace. However conducted, the roasting operation usually' leaves a small percentage of sulphate sulphur in the product. With some ores this sulphate may be troublesome in the next step, 40

unless the roasted prod'uctis allowed'to "season, that is, to stand a short time, say 24 hours, and

to cool off. Freshly roasted'ore containing appreciable quantities of sulphates, unless "seasoned, is apt to develop considerable heat when moistened for the succeeding steps, and to "set", somewhat as concrete does, and thereby to interfere with the treatment. This seasoning is not alwaysnecessary but, as stated above may be desirable in certain instances.

as to furnish a slight excess of fluorine above 66' the amount calculated as chemically equivalent to the cadmium, or to the cadmium plus lead andcopper, if these latter metals are to be removed from the ore at this stage. The fluorine salt should be groundsufiiciently fine so that it can be intimately and uniformly mixed with the ore. In treating most ores a quantity of fluoride salt small in proportion to the amount of ore present is used, so that if large particles of the fluoride are employed it will not be possible to effect the requisite intimate mixture with the ore particles and, as a result, some of the cadmiumnfineral particles in the ore mass may be so separated from the particles of fluoride that the reaction will be incomplete.

The, mixture of roasted ore and fluoride salt (together with additional fine fuel, if the roast contains insuflicient sulphide sulphur to furnish the heat necessary for the sintering operation to follow) is now moistened slightly and thoroughly mixed to distribute the fluoride (and. fuel) uniformly throughout the mass. Thismixing is important, for various reasons, and should be carefully done. The apparatus known as the pelletizer-mixer is especially suitable for this operation, since it retains the materials for a considerable period of time,- constantly rolling and beating the particles-together, and finally delivers them as a stream of uniformly mixed material in the form of loose, non-compacted particles well adapted to the next-treatment. The proportion of moistening water is highly important, as either too much or too little is harmful. The mass should be moist enough barely to hold its shape when squeezed together lightly in the hand,

and when properly moistened small pellets will be noticeable in thestream leaving the pelletizermixer. But as the correct percentage varies considerably with the difierent ores, no hard and fast percentage can be stated, the physical effect being the better'criterion. It will-be understood also that for a given ore, when the proper proportions of fluoride salt and water are known,

the fluoride may be added in the form of a water.

solution, the requisite amount of fluoride for a definite quantity of ore being dissolved in the quantity of water needed to moisten that ore to sible to measure accurately the temperature at- I 1 tained, but it is believed to be in excess of 1000 C and possibly in excess-of 1200 C.

During the sintering operation, thus conducted, reactions occur in the bed, the exact nature of which is not definitely known but which result in the formation of metal fluorides which are volatile at the high temperature attained andwhich are swept away with the gas stream. Of

the several metals ordinarily present in the ores,

cadmium seems to be the most readily acted upon, followed by lead and copper, while zinc, even when present in large proportions, is but little affected unless a great excess of fluoride above the chemical equivalent of the other metals is employed. Calculations based on the heats of reaction of thevarious compounds indicate that theoretically the zinc should react with the fluorine as readily as or even before the other metals, and that cadmium should be affected last; but for reasons not yet apparent, in actual practice the contrary-result has been found to occur and to afiord a simplemethod of separating the cadmium from the zinc in the ore. It-is not meant that the various reactions each pro-.

ceed to absolute theoretical completion before the next one starts, but the separation is complete for practical commercial purposes.

Thesintered product delivered from the sintering machine contains the zinc, but is practically free from cadmium (and from lead and copper, if suflicient fluoride salt was used to react with these metals). Consequently a high grade zinc metal may be obtained from it by .the retort method, one comparable to'the metal obtained by the electrolytic method and commanding the same premiumover ordinary zinc metal. As the retorting of the purified zinc sinter is done by standard methods, however, it is not described further.

Before proceeding to a, description of the treatment of the volatilized metal compounds it may be stated that, if desired, the preliminary roasting operation hereinbefore described may be omitted. In such case. the comminuted raw ore, with the requisite amount of fluoride salt and with fuel if necessary, -.may be mixed, moistened and blast roasted as above described, the cadmium, etc., being volatilized as in the case of pre-roasted ore. In general, however, pre-roasting in hearth furnaces would be preferable since this is a cheap method of removing a large part of the sulphur from the ore and is especially adapted .to the manufacture of sulphuric acid, which is a valuable by-product for plants in most localities.

Turning now to the treatment of the volatilized metal fluorides in the gases from the sintering operation, these are condensed as the gases cab], and may be collected as fume in a bag house, Cottrell precipitator or other fume collecting device. Besides cadmium, the fume will usually contain some lead and copper and a little zinc. This fume is preferably mixed with sulphuric acid 'inamount slightly in excess of that chemically equivalent to the several metals, and then heated to convert the metals to sulphates. This operation should be conducted in a closed apparatus provided with a vent through which the liberated hydrofluoric acid'or other volatile fluorine compounds may be drawn oi. the gases being treated with limewater or'soda solution to regenerate the corresponding fluoride which maybe recovered by filtration orevaporation, or used again in the treatment of more ore.

The mass containing the metal sulphates is then leached with water, preferably hot, to dissolve the soluble sulphates, and filtered to separate the insoluble residue. This latter contains I the lead previously volatilized from the ore. It

may therefore be dried and smelted to recover this metal.

This latter method may be preferable in small plants or where the expense of an electrolytic plant is not warranted.

The residual solution, containing the zinc as sulphate and with other metals removed as above described, may itself be electrolyzed to deposit the zinc, but in most cases the small amount of zinc present may more economically be recovered by evaporating the solution and returning the dried zinc sulphate, with fresh ore, to the roasting furnace .or to the sintering machine.

As illustration of the separation eflected by the herein described process, the following exam ples are given. A certain zinc orecontaining 0.77% cadmium and 0.1% lead was mixed with fluorspar (calcium fluoride) in the proportions 100 lbs. ore to 2 lbs. fluorspar and treated as described herein. The resulting sinter contained 0.07% cadmium and 0.02% lead. Another charge of the same ore was mixed with sodium fluoride in the proportions 100 lbs. ore to 2 lbs. sodium fluoride, and treated similarly. The slntered product was'found to contain 0.08% cadmium and 0.13% lead.

While calcium and sodium fluorides have been used as illustrative of suitable fluoride salts,,it will be understood that other alkaline earth metal and alkali fluorides may also be employed.

plant. It further provides a ready method, forrecovering the various metals separated from the ore which can be adapted to'existing plants with relatively little expense, thus afiording recovery in marketable form of products thatare ordinarily lost or discarded. It requires only inexpensive reagents, and these may be recovered for cyclic use by methods shown herein.

l The fluoride fume obtained by my process has the advantage as compared with other fumes,

such as chloride fume, in that it is not deliquescent, and whereaschloride fume tends to absorb moisture on standing, the fluoride fume remains dry and is readily removable from the fume recovering means. This is of decided advantage inasmuch as in commercial operation the fume is allowed to accumulate over a considerable period of time. The dry character of the fume avoids deterioration and corrosion losses and makes it a more easily handled fume.

I Moreover, the quantity of fluoride fume re-' quired for a given quantity of cadmium is much comprises mixing with said mineral matter alkali or alkaline earth fluorides in amount exceeding the chemical equivalent of the cadmimn content, sintering the mixture at high temperatures whereby the cadmium is converted to fluoride and the fluoride is volatilized, removing said volatilized cadmium fluoride with the waste gases,

cooling said gases whereby the cadmium fluoride is condensed and recovering the said cadmium fluoride as fume, treating said fume with sulphuric acid whereby the cadmium fluoride is converted to sulphate and the fluoride is'liberated as hydrogen fluoride, removing. said hydrogen fluoride and treating it with lime, thereby regenerating calcium fluoride, and using said regenerated calcium fluoride to treat additional mineral matter.

' REED W. HYDE. 

